A. Field of the Invention
The present invention relates to a method for measuring the impact resistance of a polymeric film-based laminate, and more particularly to a method using a polymeric film-based laminate installed upon a subgrade substrate for resisting the rupturing impact of stones backfilled against the laminate.
B. Related Art
The impact strength of a film is a measure of its resistance to shock loading. One common method for measuring the impact strength of film is known as the falling dart method. This method employs a dart having a hemispherical head attached to a weighted body held by an electromagnet above the film sample. See e.g., Bresson, Plastic Films, 3d. Ed. (Longman Scientific & Technical, New York 1988) pp. 100-103. When released, the dart drops onto the film sample which is horizontally mounted across a hole in a table. The impact strength of the film may be calculated, for example, by dropping the dart from various heights beginning from a height which does not rupture the sample, to a height at which 50% of these samples rupture. The impact energy of the film (i.e. the resistance of the film to impacts having such energy) is calculated from the equation E=mgh wherein m is the mass of the dart (lbs), g is the acceleration due to gravity (32 ft/sec.sup.2), and h is the height (ft) at which 50% of the samples rupture such that E is expressed in terms of lbs-ft/sec.sup.2.
The present inventors have surprisingly discovered that the standard falling dart test, even when samples are mounted upon a concrete block, gives results that do not necessarily correlate with performance in the field. One particular instance in which this failure of correlation occurs is in backfill construction operations, wherein a mixture of stones (i.e., rocks, pebbles) is poured ("backfilled") against a subgrade building foundation upon which a waterproofing laminate, comprising a carrier film and waterproofing sealant layer, has been installed. It is not uncommon for backfill to contain stones large enough to rupture or puncture the laminate on impact and defeat its waterproofing purpose.